Intra-aggregate CO₂ enrichment: a modelling approach for aerobic soils

Abstract. CO₂ concentration gradients inside soil aggregates, caused by the respiration of soil microorganisms and fungal hyphae, might lead to variations in the soil solution chemistry on a mm-scale, and to an underestimation of the CO₂ storage. But, up to now, there seems to be no feasible method for measuring CO₂ inside natural aggregates with sufficient spatial resolution. We combined a one-dimensional model for gas diffusion in the inter-aggregate pore space with a cylinder diffusion model, simulating the consumption/production and diffusion of O₂ and CO₂ inside soil aggregates with air- and water-filled pores. Our model predicts that for aerobic respiration (respiratory quotient = 1) the intra-aggregate increase in the CO₂ partial pressure can never be higher than 0.9 kPa for siliceous, and 0.1 kPa for calcaric aggregates, independent of the level of water-saturation. This suggests that only for siliceous aggregates CO₂ produced by aerobic respiration might cause a high small-scale spatial variability in the soil solution chemistry. In calcaric aggregates, however, the contribution of carbonate species to the CO₂ transport should lead to secondary carbonates on the aggregate surfaces. As regards the total CO₂ storage in aerobic soils, both siliceous and calcaric, the effect of intra-aggregate CO₂ gradients seems to be negligible. To assess the effect of anaerobic respiration on the intra-aggregate CO₂ gradients, the development of a device for measuring CO₂ on a mm-scale in soils is indispensable.